The present invention relates to a homogeneous charge compression ignition engine that is switchable between homogeneous charge compression ignition and spark ignition combustion.
Japanese Laid-Open Patent Publication Nos. 2004-204745 and 2007-162527 describe examples of homogeneous charge compression ignition (HCCI) engines. The HCCI engine is advantageous in that is generates only a small amount of NOx and practically no soot. However, an HCCI engine may not be able to perform homogeneous charge compression ignition combustion depending on the driving state of the engine. Thus, as described in the above publications, an HCCI engine executes switching control to switch combustion modes when necessary between homogeneous charge compression ignition combustion and spark ignition combustion.
The engine includes a combustion chamber, the temperature of which changes depending on whether the engine is performing homogeneous charge compression ignition combustion or flame propagation combustion caused by spark ignition. The temperature in the combustion chamber is higher in spark ignition combustion than in homogeneous charge compression ignition combustion. When switching from spark ignition combustion, in which the wall surface temperature of the combustion chamber is higher, to homogenous charge compression ignition combustion, ignition may occur at an abnormally advanced timing. Further, engine knocking may occur.
Japanese Laid-Open Patent Publication No. 2004-204745 describes an engine controller that lowers the temperature in the combustion chamber by, for example, opening an exhaust valve for a longer period, opening the exhaust valve twice, or drawing the exhaust gas discharged to an exhaust passage back into the combustion chamber.
However, the engine controller described in Japanese Laid-Open Patent Publication No. 2004-204745 executes control that lowers the temperature in the combustion chamber when switching from spark ignition combustion to homogeneous charge compression ignition combustion. Such control would not immediately lower the wall surface temperature of the combustion chamber and thus does not effectively prevent abnormally advanced ignition and engine knocking.